1. Field of the Invention
This invention relates to the field of semiconductor manufacture, and more specifically to the making of gallium nitride (GaN) from the reaction of gallium (Ga) vapor with gaseous anhydrous ammonia (NH3).
2. Description of the Related Art
Gallium nitride is a well-known refractory nitride optoelectronic material from the group of III nitrides having an energy gap that covers the spectral range from red to deep ultraviolet. Among its known uses are color displays, color copying, optical storage, power and microwave electronics, optoelectronics, and radiation detection from infrared to X-rays.
One of the first instances of GaN synthesis was by Juza and Hahn when ammonia was passed over hot gallium to produce small needles and platelets of GaN. Later, Maruska and Tietjen used chemical vapor deposition to make a large area of crystalline GaN on sapphire.
For commercial GaN device applications, metalorganic chemical vapor deposition (MOCVD) has been used to produce super bright blue light-emitting diodes (LEDs).
Without limitation thereto, prior methods of making GaN include the above-defined MOCVD process, a reactive sputtering process wherein atoms or ions of a solid material target are ejected into a gas phase by a momentum exchange with energetic particles, the growth of GaN by the vapor phase epitaxy (VPE) method, and the growth of GaN by the molecular beam epitaxy (MBE) method.
VPE is a chemical vapor deposition method that is carried out in a hot wall reactor at up to atmospheric pressure wherein gallium monochloride (GaCl) is synthesized upstream in the reactor by reacting HCl gas with liquid Ga metal at from 800 to 900-degrees C. GaCl is then transported to a substrate where the GaCl reacts with NH3 at 900 to 1100-degrees C. to form GaN via the following reaction.
GaCl+NH3⇄GaN+HCl+H2
The primary precursor gases that are employed during growth of GaN by the MOCVD process are Ga(CH3)3 or Ga(C2H5)3 and NH3, while GaCl3 and NH3 are also used for the growth of GaN by the halide VPE method.
MBE is a thin-film deposition process in which beams of atoms or molecules react on a clean surface of a single-crystalline substrate that is held at a high temperature under an ultrahigh vacuum of better than 10xe2x88x9210 torr. GaN has been grown by MBE using NH3 as the source of molecular nitrogen (N2) wherein the NH3 is decomposed on the surface of a substrate by pyrolysis at from 700 to 900-degrees C., and wherein N reacts with Ga to form GaN. The growth mechanism of GaN by MBE is believed to consist of the thermal activation of NH3 and surface reaction mitigated dissociation followed by reaction of N with Ga to form GaN.
Reference can be made to the two publications entitled xe2x80x9cGallium Nitride (GaN) 1xe2x80x9d and xe2x80x9cGallium Nitride (GaN) IIxe2x80x9d, by Jacques I. Pankove and Theodore D. Moustakas, volumes 50 and 57 of xe2x80x9cSemiconductors and Semimetalsxe2x80x9d, Academic Press, copyright 1998 and 1999, for a discussion of GaN.
It is against the above generally stated background that the present invention was made.
A high quality GaN boule 60 (i.e., a carrot-shaped and generally synthetically formed mass of GaN having the structure of a single crystal) is epitaxially grown by reacting a vapor of the metal Ga with the gas NH3 at a high temperature of about 1200-degrees C., which high temperature causes the NH3 to dissociate into the two elements N and H.
While NH3 is know, as a result of measurements made by the present inventor, to require a temperature of about 1400 degrees C. in order to produce a dissociation of the NH3, in accordance with the present invention the presence of Ga vapor acts as a catalyst that lowers the dissociation temperature of NH3 to about 750-degrees C.
In accordance with this invention, a seed 51 of GaN is placed within a growth furnace 30 that is heated to about 1200-degrees C. An input stream of Ga vapor and an input stream of NH3 gas are then incident on the GaN seed. Again as a result of measurements made by the present inventor, it was determined that GaN does not decomposed at 1200-degrees C. in the presence of NH3.
Ga is a solid at room temperature, Ga becomes a liquid at about 30-degrees C., and Ga becomes a vapor at the temperature that is within the growth-furnace.
An upward-facing, shower head-shaped, manifold 10 of unique construction is provided to uniformly distribute the Ga vapor and the NH3 gas to the interior of the growth furnace at a location that is generally below and spaced from the bottom of the GaN seed, for example, spaced by a distance of from about 0.5 to about 10 mm. GaN vapor is thus formed within this space, generally adjacent to the surface of the boule.
Ga vapor is flushed or carried into the growth furnace by way a controlled stream of nitrogen (N2). At the location of the exterior surface of the GaN seed, the Ga vapor reacts with the NH3 gas to epitaxially form solid GaN on the exterior surface of the GaN seed, and to also form H2.
While it is possible that some GaN vapor forms within the growth furnace, and then deposits epitaxially on the GaN seed, it is believed that most of the solid GaN epitaxially forms on the GaN seed from a combination of the Ga vapor with the N that results from the decomposition of the NH3. In other words, most of the Ga vapor moves away from the manifold and toward the surface of the GaN seed, whereupon the Ga vapor combines with N and then deposits on, and thereby epitaxially grows, the boule.
The manifold flat top surface 11 includes a plurality of physically-spaced nozzles 13, each nozzle having a generally circular-shaped orifice. A heated chamber 21 is formed directly under this top surface, and this heated chamber includes the nozzles. Liquid Ga is compressed, using a motorized pipette 39, and the compressed Ga vapor is then piped, under pressure, into the nozzles, along with nitrogen. Within the hot nozzles the Ga vaporizes, as the Ga vapor is mixed with a controlled amount of nitrogen gas.
Gaseous NH3 is also supplied under pressure to portions of the hot chamber that surround the Ga nozzles, so as to flow through areas 131 of the manifold top flat surface that surround each Ga nozzle. As a result of the relatively high temperature of about 1200-degrees C. that exists at and above the manifold""s top surface, GaN vapor moves upward toward the exterior surface of the GaN seed.